Various angioplasty techniques have been in use for several years. Typically, a catheter is introduced into the body through an artery in the leg or arm and threaded into the artery or blood vessel that has restricted blood flow due to the build-up of atherosclerotic plaque. The most common technique in current practice is balloon angioplasty. The catheter positioned within the subject artery has a deflated balloon at its tip. The balloon is inflated within the artery and the expansion of the balloon is designed to "press" the plaque into the artery wall, thereby minimizing blood flow restrictions. Balloon angioplasty generally just manipulates the form of the plaque, and does not create a significant problem of plaque residue flowing from the site. Unfortunately, balloon angioplasty has several failings and a relatively high complication rate.
Atherosclerotic plaque build-up can exist in a number of different forms. The plaque can be quite hard and scaly or more fatty and pliable. The areas of plaque accumulation are generally not symmetrically located at a particular point in the artery, rather adhering to only portions of the artery walls.
Considerable efforts have been directed toward finding improved means to perform angioplasty procedures. Numerous devices recently have been described that utilize the application of heat to resolve atherosclerotic plaque. See for example, U.S. Pat. No. 4,654,024 of Crittendon et al. and U.S. Pat. Nos. 4,748,979 and 4,672,962 of Hershenson. The most extensive research concerning the use of heat to resolve atherosclerotic plaque has been directed toward the area of laser angioplasty techniques. In most laser angioplasty devices the laser is used simply to supply heat to the tip of the catheter. See for example, U.S. Pat. No. 4,784,133 of Mackin; U.S. Pat. No. 4,685,458 of Lechrone; U.S. Pat. No. 4,770,653 of Shturman; U.S. Pat. No. 4,662,368 and 4,773,413 of Hussein; and U.S. Pat. Nos. 4,732,448 and 4,641,912 of Goldenberg.
The various angioplasty techniques described in the literature uniformly fail to address the asymmetric disposition of the plaque within the artery. In most cases, the tip of the angioplasty catheter acts as if the plaque consists of a uniform symmetric coating on the interior wall of the artery. Particularly in those techniques which use something other than pressure to manipulate the plaque, the resolving forces are applied indiscriminately to the plaque and to the healthy tissue within the artery.
Radio frequency sparking to cut or cauterize tissue as a medical procedure is common in the prior art. There are two basic classes of electrosurgical devices. Monopolar devices consist of a high-frequency electrical (generally RF) generator, a cutting or cauterizing electrode or needle, and a patient plate. The patient plate is attached to the body of the patient, and acts as the return electrode for completion of the RF circuit. Cutting occurs due to the heat generated by RF sparking from the electrode to the patient's body tissue. The shape of the electrode concentrates the RF energy, thus creating the high temperature spark. Appropriate modulation of the frequency determines whether cutting or cauterizing will occur. The relatively larger surface area of the patient plate, which is in contact with the patient's body, prevents the current flow from concentrating at one point. This prevents the RF current from burning the patient as the current exits the body.
There are also several bipolar electrosurgical devices described in the prior art. Bipolar devices consist of a high frequency electrical generator and an instrument that contains both the delivery and return electrode. RF sparking occurs between the two self-contained electrodes of the instrument. The bipolar electrosurgical devices of the prior art are generally inadequate due to the conditions necessary to create bipolar sparking. The most fundamental difficulty is that bioactive electrodes must have a roughly equal voltage drop at both the delivery and return electrodes. The high power current required in order to achieve bipolar arcing often causes extraneous sparking, particularly when there is unequal contact with the surrounding tissue.
The extension of known electrosurgical processes--utilizing RF sparking--to angioplasty techniques is relatively unexplored. A disclosure of a monopolar electrosurgical catheter for use in resolving atherosclerotic plaque is found in U.S. Pat. No. 4,682,596 of Bales. The mono and bipolar devices in Bales describe a hollow catheter with a hollow tip member. Only one potential electrode, at the catheter tip, is envisioned by the Bales patent. Bales briefly describes the utilization of variously modulated waveforms in order to resolve atherosclerotic plaque, and the application of high power levels while minimizing the creation of excessive amounts of heat. However, means are included for removing residue from the plaque destruction site, indicating that the modulation techniques employed have not been maximized. It should be possible to destroy the plaque in such a manner so as to eliminate significant residue formation.
An article by Cornelius J. Slager et al. in the Journal of the American College of Cardiology entitled "Vaporization of Atherosclerotic Plaque by Spark Erosion" (June 1985, pp. 1382-6) describes the use of a bipolar RF sparking catheter. Again, there is a single spark generating electrode. The sparking frequency is modulated, but not to optimize ablation. Synchronous transmission of energy with cardiac contraction is employed in order to minimize the disruption of electrical pathways in the heart.
U.S. Pat. No. 4,643,186 of Rosen describes an "antenna" type bipolar RF sparking catheter for use in angioplasty. The delivery and return electrodes are configured in such a way that the electrodes terminate together to form an "antenna."
When current is supplied to the antenna, RF sparking will occur. The addition of balloon means encapsulating the sparking antenna is also described. Rosen discloses coating the interior surface of such balloons in order to supply some control over the direction of sparking. Such directional manipulation can only be accomplished before the catheter is introduced into the patient's body. No means are disclosed for directing the random sparking of the "antenna" once introduced into the desired artery.
An example of an asymmetrically shaped electrode or energy applicator is seen in U.S. Pat. No. 4,311,154 of Sterzer. The Sterzer patent discloses a device to be used in the treatment of a cancerous tumor with high temperatures, or hypothermia. Sterzer describes a device for hypothermic treatments utilizing microwave energy so that heat radiates nonsymmetrically from the surface of the instrument. Sterzer does not utilize RF sparking and, like the Rosen patent, does not contemplate the use of means for directing the energy once the device is in place within the body.
The examples discussed above where RF sparking has been used for the resolution of atherosclerotic plaque employ relatively unsophisticated means. The RF spark is a very powerful and intense force to be let loose within the human body. Means for effectively harnessing the vast potential of RF sparking angioplasty have not been disclosed prior to this invention.